1. How strong are Coulomb forces? Electron and proton in a hydrogen atom Compare electric and gravitational forces electron and proton m e = 9.11x10 -31 kg, m p =1.67x10 -27 kg 1

2. Question A Human weight 120 lb, which of the following is correct? a)A large fraction of the weight come from the attraction force between the charges on human body and earth. b)All the weight comes the attraction force between the charges on human body and earth. c)All the weight come from the gravitational forces. The electric forces are negligible. 2

3. Two point charges are separated by distance d as shown. Where can you put a third charge of +1 C so that there is no net electric force acting on it? (Take Q > 0.) e = 1.6  10 -19 C k = 8.99  10 9 Nm 2 /C 2 a) to the right of charge -Q b) to the left of charge 2Q c)  between the two charges d) some other place e) nowhere 2Q - Q d 3

4. Principle of Superposition of Electric Force q1q1 q2q2 q3q3 F 13 F 12 F 14 Add by components or Magnitude and direction separately by using trigonometry q4q4 4

5. Two positive charges, one 2  C and the other 7  C, are separated by a distance of 20 cm. What is the magnitude of the electrostatic force that each charge exerts upon the other? a)0.32 N b)0.63 N c)0.70 N d)2.02 N e)3.15 N 5

6. Three positive charges are located along a line as shown. What is the net force exerted on the 0.02-C charge by the other two charges? a)2.25 x 10 6 N b)4.5 x 10 6 N c)9.0 x 10 6 N d)1.8 x 10 7 N e)2.7 x 10 6 N 6

7. The Electric Field How do the charges exert forces on each other, when they are not even touching? – The concept of an electric field describes how one charge affects the space around it, which then exerts a force on another charge. – The electric field at a given point in space is the electric force per unit positive charge that would be exerted on a charge if it were placed at that point. 7

8. We can then use the field to find the force on any other charge placed at that point: – The direction of the electric field is the direction of the force exerted on a positive test charge. – If the charge q is negative, the minus sign indicates that the direction of the force on a negative charge is opposite to the direction of the field. – We can talk about the field at a point in space even if there is no charge at that point. – The electric field can exist even in a vacuum. – The field concept can also be used to define a gravitational field or a magnetic field, as well as others. 8

9. Principle of Superposition for the Electric Field q1q1 q2q2 q3q3 E3E3 E2E2 E4E4 Add by components or Magnitude and direction separately by using trigonometry q4q4 9

10. Electric Field Produced by a Point Charge 10

11. Are Q1 and Q2 positive or negative ? Q1 Q2 A.Q1 is positive, Q2 is negative B.Q1 is positive, Q2 is positive C.Q1 is negative, Q2 is negative D.Q1 is negative, Q2 is positive 11

12. An electric dipole is two charges of equal magnitude but opposite sign, separated by a small distance. – Electric field lines originate on positive charges and end on negative charges.  The field lines point away from the positive charge, and in toward the negative charge. 12

13. Two charges, of equal magnitude but opposite sign, lie along a line as shown. What are the directions of the electric field at points A, B, C, and D? a)A:left, B:left, C:right, D:right b)A:left, B:right, C:right, D:right c)A:left, B:right, C:right, D:left d)A:right, B:left, C:left, D:right e)A:right, B:left, C:right, D:right